Presently, our system shows one less collaborator on your Collaborators page than you actually are using.
Although you see the current number of collaborators on your account, the account owner (creator & administrator) is also included as one of the collaborators, but is not shown on the list. We are working hard to clarify this more clearly in our user interface, so we apologize for the temporary confusion.
If your plan allows only 10 collaborators, and you see 9 on the list, if you try to add one, you will find that the system will not let you. That is because, once you count the administrator (account owner) who is not shown on the list, you are really at 10 collaborators already. So you will have to either upgrade your plan, or remove some collaborators.